Method of dispersing carbon black in bulk rubber



' Jan..`2l, i958 F. B. SMITH. 2,820,838

METHOD 0F DISPERSING CARBON BLACK 1N BULK RUBBER 'Filed March 3o, 1954 2 sheets-:snaaiI 1 Jm. 21, 195 F. B. SMITH 2,820,838

METHOD OF DISPERSING CARBON BLACK IN BULK RUBBER Filed March 30, 1954 2 Sheets-Sheet 2 AGENT United States Patentl C NIETHOD OF DISPERSING CARBN BLACK 1N BULK RUBBER Frank B. Smith, Detroit, Mich., assignor to United States Rubber Company, New York, N. Y., a corporation of New Jersey Application March 30, 1954, Serial No. 419,746

5 Claims. (Cl. 260-763) This invention relates to an improved method of dispersing carbon black rubber, and more particularly it relates to an improved method of achieving microscopic dispersion of carbon black in rubber.

One object of the invention is to obtain an improved state of carbon black dispersion, particularly in rubber tire tread mixes.

Still another object is to obtain a faster rate of dispersion of carbon black in bulk rubber.

It is yet another object of the invention to reduce the power requirements for mixing carbon black with bulk rubber.

Still a further object of the invention is to obtain more effective use of mixing equipment by reducing the length of the mixing cycles in Banbury mixers and related equipment.

It is still a further object of the invention to produce rubber tire tread mixes of improved quality.

The relation existing between resistance to wear and degree of dispersion of the reinforcing agent, namely, carbon black, in rubber articles such as tire treads is well recognized among manufacturers of rubber goods. Briefly, when the dispersion of the carbon black is excellent, say below 1% undispersed carbon black as observed under the microscope, then tread wear is very good. When the dispersion of the carbon black is poor, say above undispersed carbon black, the resultant tread wear is apt to be impaired.

In conventional processes, a considerable amount of work is expended in an effort to disperse the carbon black. In tire practice, two or more mixing stages are usually specified, using initially an internal mixer, such as a Banbury mixer, followed by a train of three or four roll mills to which the stock is successively transferred to complete the mixing. Usually the carbon black only is incorporated in the rubber in the Banbury, to form a masterbatch, to which the other compounding ingrey dients, such as vulcanizing agents and accelerator, are added at a later mixing stage on a roll mill or in a Banbury mixer, and it is in such later mixing stage that the final state of dispersion of the carbon black is obtained.

The conventional mixing procedure therefore represents a substantial item of expense in rubber processing, since much power is consumed, the equipment is extremely heavy and expensive to install and maintain, and the necessarily long mixing cycles very much limit the productivity of this expensive equipment. Furthermore, in spite of the strenuous mixing efforts, the degree of microscopic dispersion of carbon black obtained in practice is extremely variable and leaves much to be desired.

There are at least two approaches to the problem of improving the mixing and rendering it more economical. The first possibility is the re-engineering of present equipment or the design of new machinery. In this work, one generally adheres to established principles, seeking a way of more effectively applying energy to the mix. My in- .ventina is based on, a second approach, which-in con- 2,820,838 late'nted Jan. 21, 1958 ice t accompanying drawing, wherein:

Fig. 1 is a ow diagram representing a conventional vtire tread stock mixing procedure;

Fig. 2 is a fiow diagram representing a procedure of the invention;

Fig. 3 represents a microphotograph of a rubbercarbon black stock made by a conventional mixing process;

Fig. 4 is a similar view of a stock mixed in accordance with the invention; and

Fig. 5 is a graphical representation of the effect of concentration of alkanol on the amount of undispersed carbon black.

Before stating the principle of my invention, it should first be explained that I have observed that when carbon black is mixed with rubber, the carbon black is first compacted or agglomerated into large aggregates or flakes, due to the compacting action of the mill. This effect is noted on the first pass through the mill nip. It has been observed that these masses are difficult to disperse and largely lacking in affinity for the rubber matrix. Prolonged milling is necessary to incorporate and disperse the cohesive flakes of carbon black into the elastomer. My invention is based upon the discovery that if a definite amount of an alkanol having not more than four carbon atoms is present when the carbon black is initially masticated with Athe rubber in an internal vmixer, this compacting or agglomerating effect does not Approxi- Pounds per mate Percent cubic ft. specific air gravity Original condition 3. 0 0. 05 97. 0 Uncompressed... 13. 4 0. 21 88. 0 Semi-compressed- 2l. 1 0. 34 81. 0 Heavy-compresse 25. 0 0. 40 77. 8 Beads 27. 0 0. 43 76. D

The so-called dustless or compressed carbon black (third and fourth grades, above), and especially the pelletized carbon black (beads) of high density,vis employed in my invention.

In practicing the invention I typically first mix the dry carbon black (i. e., air-dry carbon black usually containing only a very small amount of moisture, such as some 1% moisture) with a definite amount of an alkanol of not more than four carbon atoms within a certain critical range. For purposes of the invention there must be added to the dry carbon black at least about 25 parts of the alkanol per parts of the carbon black, and preferably somewhat more of the alkanol than this, say 50 to 160 parts, is used. However, in no case need the amount of the alkanol exceed 250 parts per 100 parts of carbon black. The preferred alkanol for use in the invention is ethanol, conveniently in one of its commercial denatured forms, although methanol and the propano!! and butanols may also be used.

- Mixing of the carbon blackwith the alkanol contain- 2,820,831; Y Y e j s ing'anot more tbanfour czarbon atoms is suitably effected-:in amixer capable of exerting substantial shearing action, such as a Werner-Peiderer mixer or a colloid mill, and in the course of the mixing the carbon black pelletsorl granules (such vrbeing the typicaliniti'al for'm l of the carbon black used in the process) a'r'eE disintegrated. Mixing periods of 25 -to Y 10"'minute's aeage'nerallyadequate Ycarbon blak at this stage-'since thespecitiedamounts of -alkanol mustV be presentlin the carbon black'whenitis subsequently mixed with the rubber.

The mixture of .the alkanol containing l-not-m'ore than four carbon atoms prepared as described is thencha'rged vto a heavyduty guml rubb'er'mixer ofthe Banbury type along with from 150 to 250 parts of rubber, per v100 parts of the carbon black, and the whole is subjectedto severe mastication orv shearing action. As a consequence ofthe presence of the -specied amounts'of 'the a'lk'a'nol along with the carbon black, the dispersion 'of the carbon black in the rubber is greatlylfacilitated, in comparison to convntionalprocedure. To understand the remarkable'eect produced by the invention, it yshould first be recognized that there is a denite distinction 'between the conceptlof incorporation of the'carbonblack and `4the conceptof dispersion of the carbon'black, although these terms are sometimes used looselyyand in-fa'ct inter- -changeably, in the prior art. The ditfe'rence is"clea`r if one examins the rubber mixture under the microscope.V

Carbon black can be incorporated, yet not dispersed. The mixing process is roughly comprised of-th'etfollowing successive stages:

(1) Incorporation (absorption of the c'a'rboublackby the rubber).

(2) Homogenizationv(gross dispersion).

(3) Dispersion (microscopic and sub-microscopicinixing of the carbon black aggregates "and particles).

4In phase 1) the carbon'black is absorbed bythe rubber. In phase (2) the carbon black -is distributed inlthe rubber matrix more or less uniformly. In the `absorption process small amounts of rubber pick up large "amounts of the carbon black. Redistribution occurs with continued mixing in phase (3) and the carbon black is tinally dispersed as microscopic and sub-microscopic particles or aggregates. The distinguishing feature of my process is that it` givesa remarkably faster rate of microscopic dispersion, o'r an improved degree of microscopic dispersion, depending upon-how one wishes to utilize the process. Thus, by masticatin'g vIthe alkanol-carbon Iblack-'mixture with the ig'm rubberlfor normal Amixing "timesfone 'obtai-ns a far fs'uperior ldegree of microscopic vclispersionthan in-conventional-processes,or one can mix for considerably less than the conventional mixing time and still obtain a; degree Yof microscopic dispersion that is as good as that obtained byrconventional practices ftaking a 'much longer time. Y

-It should -be emphasized here that the effect of the specified amounts'ofY the alkanolof four vcarbon atoms or less in admixture with the carbon black is unlike the eiect that would be obtainedin the presence of liquids having a swelling or solvent action on the rubber. Liquids v having .a solvent or vswelling-action, such as oils, benzene,

etc., may ,produce'a more rapid incorporation (distinguished-from Vtrue microscopic dispersion above) of the carbon black in *the rubber, and therefore upon supericial observation it might be'fconcluded that such liquids are aidingthe-mixing Loperation, Vbut I vhave determined by microscopic vobservation -Ithat-.thefactual Vstate of dispersionbf the-carbon'blaek-is not improved by the liquids which do swell rubbery greatly, incontrastto the CLI course devoid of swelling action, actually lead to much poorer microscopic dispersion, and they are therefore undesirable.

In place of preparing the mixture of carbon black containing the required amount ofthe alkanol of the required class beforehand in a Werner-Pfleiderer mixer or the like and adding this mixture to the bulk. rubber in the Banbury, Imay-inste'ad prepare the vcarbon Ablackalkanol mixture in the jBanbury before or after adding the bulk rubber thereto. In any event it is essential that the specified amount of the 'alka'nol-be present when' the carbon black is initially masticate'd vwithfthe 'rubber'.

It is important to note that the beneficialetfects'f'the present invention' are obtainable only "when `the 'alkanoL carbon black and rubber are mixed under confinement in an internal mixer, where'the mix Yispositively maintained under pressure in engagement with the masticating blades of the mixer. The desired effect is not obtained V'on an open roll *mixe'ryvvhere the yalkanol-'carbonrA black would merely be'squeezed outof lthe rubber as the mixture Lpassed through the nip of the 4mill rolls. Y

After the alkanol, ycarbon black 'and rubber l'ia've been ma'sticated together in the internallmixerfsuicient- 'lyltoprodu'ce-a suitable degree of microscopicdispersion, it is necessary to remove the greater part of'fthealka'nol from the mix. VI typically'accomplishthisbyexpressing the alkanol out of the '`mi`x, suitably y'by=dis :har'gin`g the mixture f'rom the-Banbury onto a'squeezeroll or-rother expressing-device, wherein the alkanol is removedfrom the rubber by application of pressure. Devices suitable orlthispurpos'el'arefkn'own, and ymay be of the type shown, for -example,in U. S. Patent 2,371,722. Because ofthe fact'thatthe alkanolis thus expressed and removed from the mixture, there should not be employed, previous to this stage, any rubber compounding ingredients which are soluble in the Ialkanol to any substantial extent, since such alkanols`oluble ingredients would of course be lost when the alkanol is' expressed from the mixture. *'If alkariol-soluble compounding ingredients are'present, the alkanol should be removed by evaporation, in'order to avoid loss of such 'compounding ingredients.

Referring to the idrawing', and in particular to'Fig. A1, it will be -observed that in'the usual conventional factory practice, the carbon black and rubber are t'rst mixed together in an initial mix 1known as ia^m'asterbat1'ch, and such mixing typically leavesl some ySil-60% lf thef'carbon black -in an undispersed' form`Y (as'm'easured byV thefproceduredescribedinfExample l,V below). AThe 'nextstep'is to continue the'mi-'xjin'g in'theiBanbury while adding the remaining 'compounding ingredients,` whereupon the; undispersed carbon black is reduced-to'about-ZOWQ. The dispersion ofthe carb'o'n'black iss'till far'from complete, and it is `n'ot'until lthestock is further mixed onroll mills,lprior t-o bein'gin'tro'ducd to thetubersorextruders, 'that th`e1-undispe`rsed Ic'sa'rbon'black `is reduced vt0 anywhere near anacceptable level.

In contrasttofthefv foregoingyhemethod 'of the inventionas' shownin'Fig. 2, typiclly'e'mplys a *brief preliminary mixing ofthe carbon `black andthe alkanol of four carbon atoms or less, and va""subse'quent 'mixing for only some sixminutes inthe Banbury 'is'suicient to'reduce the'undispersed Vcaibonl'bla'ckto an unusually -low and more than ade'q'uate level, feveniwi'thout any preliminary masterbatching. lAfterfrtetrioving Ithe lalkanol from the stock, the `ba'tchis incnditi'ont'o be charged directly to the tu'b'e'rs without 'any'L necessity f'for :an intermediate line of roll mills.

The-method of-the inventiontherefore lsinflplitesifactory mixing practice considerably, while at the -same timer-making possible improvements Yin quality. The'e'con'- omies made-possible bytheinvention will beYpar-ti `Vularly evident `fromfa-*comparison `of the necessary time'swhih thefstock rnust spend in theA Banbury 'mixer-lin ftlielemventional and in the present-process. In"the'pre'se'n'tlproc- -ess the necessary-time-inthe Banbury'mixerlis verynch reduced, and therefore the productivity of a Banbury mixer is greatly increased, or a given rate of productivity may be achieved with fewer or smaller Banbury mixers. Since the Banbury mixers are extremely heavy duty machines that require a substantial initial capital investment and that are expensive to maintain and to operate because of the large power consumption, it will be apparent that this feature of the invention represents a substantial economy. Similarly the possibility of dispensing with the usual additional roll mills, prior to the tuber, represents a further economy. These economies more than olset the comparatively small expense of the additional machinery used in the present process to make the carbon black-alkanol mix and to remove the alkanol, since such equipment is comparatively lighter and less expensive to install and operate.

The following examples will illustrate the practice of the invention in more detail. In the examples all parts are expressed by weight.

EXAMPLE I To establish a standard of comparison for demonstrating the improved dispersion obtainable by the method of the invention, a test stock is first mixed by a conventional procedure, without using the alkanol of four carbon atoms or less. In this test stock 100 parts of carbon black is mixed with 195 parts of smoked sheet rubber along with about l1 parts of stearic acid in a size B laboratory Banbury mixer operated at a slow rotor speed of 75 R. P.v M. for a 31/2 minute mixing cycle to make a carbon black masterbatch. The masterbatch is then sheeted out on a roll mill. To this masterbatch additional conventional compounding ingredients including the usual small amounts of sulfur, accelerator, zinc oxide, and autioxidant are then added, and a final mixing cycle of two minutes duration is carried out in the Banbury mixer. The stock is then discharged from the Banbury and sheeted once through a laboratory mill set for 0.100 gauge. The percent undispersed carbon black is measured in samples of the mixed stock by observing thin slices of the stock under the microscope at a magnification of 400X. Undispersed black appears as opaque llake fragments or aggregates in contrast with comparatively transparent adjacent areas wherein the carbon black is well dispersed. By projecting the microscopic image onto a calibrated screen, the area of the undispersed carbon black is determined, and, knowing the percent carbon black in the sample and the area and thickness of the sample, as well as the specific gravity of the carbon black and of the stock, the percent undispersed carbon black is readily calculated. The percent undispersed carbon black may be defined as that portion of the total carbon black in a given stock in the form of fragments measurable at a magnification of 400X.

To demonstrate the invention, there is then prepared a paste consisting of 100 parts of carbon black and 160 parts of a commercial denatured ethanol sold under the trade name Shellacol by mixing these materials for minutes in a Werner-Pfleiderer or Baker-Perkins mixer. This paste is then mixed with about 195 parts of rubber and l1 parts of stearic acid in the same manner as the standard test mix described previously. After the batch has knitted together the bulk of the Shellacol is drained out by opening the discharge side of the Banbury. The

v stock is sheeted out on a mill and additional drying of the stock is effected by heating in a hot air oven overnight at 225 F. in this experimental stock, although on a commercial scale this step could be dispensed with by using eicient equipment to express almost all of the Shellacol from the stock mechanically, and relying on the heat applied in a final brief milling operation to complete the drying if necessary. The thus-dried stock (masterbatch) is then compounded with the usual vulcanizing ingredients by mixing for 2 minutes in the Banbury, in the same manner as the control stock.

The comparative results of such standard test mix and the mix of the invention using the commercial denatured ethanol are shown` in the following Table I.

Table I COMPARISON OF CONVENTIONAL PROCEDURE WITH PROCEDURE OF INVENTION Test A Test B Composition of paste:

MPC #3 carbon black 100 Shellacol Composition of masterbatch:

Plasticated smoked sheet 195 Paste of carbon black and Shellacol MPC #3 carbon black Stearic acid l1 11 Undlspersed carbon black: After masterbatching percent.- 47 0. 9 Power consumed in masterbatching kilowatt-hour.- 0. 24 0. 15 Physical properties: After curing 45 min. at 45# steam 300% modulus (p. s. i. 1, 650 1, 430 Tensile (p. s. i.) 4, 200 3, 800 Elongatiou, percent-. 565 565 212 F. tear, lbs 1. 9 4. 4 Torslonal hysteresis. 286 F- 10` 085 Relative abrader wear 116 135 Undispersed carbon black: In nal stock percent.- 11 0. 4

It will be noted that at the conclusion of the initial or masterbatching mix, the carbon black was,47% undispersed in the conventional stock, but only 0.9% undispersed in the masterbatch of the invention. This remarkable improvement is preserved and accentuated in the final stock, wherein 11% of the carbon black is undispersed in the conventional mix, whereas only 0.4% of the carbon black remained undispersed in the stock of the invention. Furthermore, in a series of such tests. it is observed that the results obtained with the conventional mix are extremely variable, whereas the results obtained by the mixing procedure of the invention are consistent and uniform. Fig. 3 shows the microscopic appearance of the conventional stock, while Fig. 4 shows the appearance of the stock mixed in accordance with the invention. The opaque areas of undispersed carbon black in Fig. 3 contrast with the uniform appearance of Fig. 4.

The physical properties of the vulcanized stocks, as illustrated in Table I, above, show that the vulcanizates processed in accordance with the invention compare favorably with those ofl the conventionally processed mixes in tensile, modulus, and elongation. The improved dispersion of the mixes of the invention is reflected in lowered hysteresis and superior abrasion resistance, as well as enhanced tearing and cutting resistance.

It is also significant to note that considerably less power was necessary to produce the superior mix of the invention in comparison with the conventional mix.

Similar results are obtained by substituting in the foregoing example the other alkanols of four carbon atoms or less. This is in direct contrast to the results obtained if one attempts to substitute higher aliphatic alcohols in the process. The higher aliphatic alcohols in similar tests have been found to produce no improvement in microscopic dispersion of the carbon black.

EXAMPLE II In this example the effect of varying the quantity of the alkanol of four carbon atoms or less is demonstrated. Pastes of 100 parts of carbon black MPC #3 containing varying quantities of Shellacol as indicated in Table II, below, were mixed as described in Example I and then added to 195 parts of rubber in the Banbury mixer, where the mixture was worked for 31/2 minutes. The amount of the carbon black in the undispersed condition was then fetermined by the mcroscnpetest lesrcribesiA ,with the-re Sslsshqwn ililTafble 111 i T able IIIV Test y O' D E F G H rmsshellacol m iooparts A carbon black None 32 v 67 92 113 130 Percent Shellacol in total` batch None 12 25 34 42 48 Percent undispersed carbon black 70.0. 45.6 1.02 0.68V 0.99V -.89V

T he data of Table II are shown in graphicalforrn in-Fig. 5.. It will be observed that a most remarkable reduction in the amount of undispersed carbon black occurs suddenly when about 25-30 parts of thei-alkanol (in 100 parts..

rubber takes place.

EXAMPLE 1n- Inordervto contrast the ,-ettlect of theiuse Yof the.-a1lgano1 of tourcarbon atomsror less in accordancepwith the inventionhonthe dispersion ofcarbon blaclcwith theundesirable eect-vof lmaterialswhich haveaA solvent or swelling/ action on"k rubber, ExampleI was'repeated, using a paste made withcarbontetrachloridginplaceofthe alkanol.- After abminute initialjnx, followed bya2f` minute nal mix,

the; ameuntofungispersed, carbomblack amounted to 90.5% This contrasts, withtheless than 1%.undispersed carbon Vblack obtainediwithlthetalkanol paste, and, in fact, itr is actually farworse.I than,- thedispersion obtained with a conventional: mix containing .noy liquid atV all.

The term .frubber; as used hereincomprehends lnoti-only natural ;rubber, but ithe f various .synthetic 'rubbers 1 known to be equivalenttheretofoiythe.purposebf makingcarbon-V blackereinforeedevulcanizates :suitable \-,fo`1'VVV tirera-treads'J and v similar mechanical rubber. goods# Having/thus described my invention,- w-hat Ic1 aim-and -r desireto protecti by Letters- Patentis In alIcases V't1ie1a1kano1niust be present when the Vmixingwof the carbon blackn withnthe 1. Y An improved method of effecting smicroscopiediss; persion. `o f"ca rb on blaclsv ina rubber, stock` comprising friasticatingfinVv an internal mixer ra.rriixtul'ecomprisingx .100 parts?,ojcompressedcarbon black, 150 to. 250p artsof rubber, androm about,v 25 to, 250 partsofvan alkanol having: from one ,to.fourA carbon atoms, and thereafter removing the :said alltanol from the mixture.

2, Anfimproved methodbf eiectingmicroscopie. dis; persionof Ycarbon-b1ack ina rubber stockk comprising 10 mastiating in anginternal mixer a` mixture comprisinglOO parts, otfcornpressedzrcarbon black, 1570-. toA 250 parts A.of-

rubber, andrornabout 50to, =160 parts oan alkanol havl ing. from. one to .fours carbon atoms,V and thereafter-rem ovingthensaid alkanoll yfrom Y themixture. Y

3 An;v ,improved methoclyor` -eiecting microscopic 'disp persion of' carbon black in a-rubber stockg comprising; masticatingin an internal mixera mixture-V comprising-100 Part-S Of'pelletizedr-carb0n; bleek; 1..50.'fof250 parts-oa ruhe ber, and from about to 250 partsfofganfallanolhaying; from one to four carbon atoms and thereafter removing the said alkanol from the-=-mixture.

4. Angfirdnproved method of effectingE miroscopiel dispersionfofl carb onfblaclcI a rubber stoclocomprising preijorrripasteof` IOOfpartsV of compressed carbon, @m505-te 160 parts. 0f ethanolraddingrthe: tem 1510 te 125,0 parts of rubber Ain.aan:i11 r11e1- .astieatiug thefsad ingredientenproduceva-2 fromy the-mixture?,

5 Ap improvedmethpdvotieectine microscopiediSr persionfg 0&5 car-beny Y' preparing@ 11.111. black and fromnabou f an internatmixer audfimastieeting the said .ingredientsgztoproduce-fatuniformrmixturey theasaidrmixture being composed= ofi' ethanolainlsoluble ingredientsgz, and.v thereafter..Y

expressing zthef ethanolnffronr thee mixture;

40' Rerereneec-irea minieme @mais parent UNTTED- STATES PATENTS OTHERs REFERENCES f Drogin et alx: 124-125.

.turen and; thereatter, remos/ine; the-ethanol;

b1ael in anrubberf stockacomprisingzpasteof 1GO parts of-:pelletizedrcarbon:

2.5 H1250 parts.-;of.-;ethan01,adding;V the said paste.; togfriomgrl 5,0 to .r;25.0 pantsr-.ofgbull rubberfin Ind," &, Eng., Chem., Feb.` 19.44,pags 

1. AN IMPROVED METHOD OF EFFECTING MICROSCOPIC DISPERSION OF CARBON BLACK IN A RUBBER STOCK COMPRISING MASTICATING IN AN INTERNAL MIXER A MIXTURE COMPRISING 100 PARTS OF COMPRESSED CARBON BLACK, 150 TO 250 PARTS OF RUBBER, AND FROM ABOUT 25 TO 250 PARTS OF AN ALKANOL HAVING FROM ONE TO FOUR CARBON ATOMS, AND THEREAFTER REMOVING THE SAID ALKANOL FROM THE MIXTURE. 